This invention relates to polarisation rotators and methods of fabricating such polarisation rotators.
V. P. Tzolov, M. Fontaine, “A passive polarisation converter free of longitudinally-periodic structure”, Optics Communications, vol. 127, pp. 7–13, 1996, discloses a polarisation rotator formed by a section of a rib waveguide having an asymmetric cross-section as a result of one side wall of the waveguide being normal to the substrate surface and the other side wall of the waveguide being inclined so that radiation propagated along the optical axis of the waveguide is subjected to a rotation about the propagation axis.
J. Z. Huang, R. Scarmozzino, G. Nagy, M. J. Steel, R. M. Osgood, Jr., “Realisation of a compact and single-mode optical passive polarisation converter”, IEEE Photonics, Technology Letters, vol. 12, no. 3, pp. 317–319, March 2000, refers to such an angled facet polarisation rotator, and describes a possible method of fabrication of such a polarisation rotator using two separate etch processes, namely a dry etching process, such as chemically assisted ion beam etching (CAIBE), for producing a vertical facet on one side of the waveguide, and a wet etching process for producing a tilted facet on the opposite side of the waveguide. The fabrication process as described in this reference uses lithographic patterning and etching steps to form the combined structure containing a tilted facet and a vertical facet. This renders the fabrication process relatively complex and low yield as a result of mask alignment difficulties, as is described in more detail below with reference to the drawings.
B. M. A. Rahman, S. S. A. Obayya, N. Somasiri, M. Ragarajan, K. T. V. Grattan, H. A. El-Mikathi, “Design and characterisation of compact single-section passive polarisation rotator”, Journal of Lightwave Technology, vol. 19, no. 4, April 2001, also provides a discussion of such a polarisation rotator.
It is an object of the invention to provide an improved polarisation rotator, and a method of fabricating such a polarisation rotator.